1. Field of the Invention
The present invention relates to a sheet conveying device, and an image reading apparatus such as a scanner and an image forming apparatus such as a copying machine, both provided with the sheet conveying device, and more particularly to a sheet conveying device used for conveying a sheet having a relatively large width.
2. Discussion of the Background
Typically, a sheet conveying device which is mounted on a copying machine, a scanner, and the like as an original conveying device to process an original sheet having a relatively large width, such as an D-size sheet in a lateral direction, has a configuration as illustrated, for example, in FIG. 7.
In such an original conveying device, when a user manually inserts an original P placed on an original table 81 into a nip portion between an entrance drive roller 83 and an entrance driven roller 84, the leading edge of the original P is detected by an entrance sensor 82. When the entrance sensor 82 detects the leading edge of the original P, the entrance sensor 82 is turned on and the entrance driven roller 84 is lifted up. After the leading edge of the original P passes through a clearance between the entrance drive roller 83 and the entrance driven roller 84, the leading edge of the original P is detected by a start sensor 86. Just after being detected by the start sensor 86, the leading edge of the original P impinges against a stopping nail 85. When the start sensor 86 detects the leading edge of the original P, the entrance driven roller 84 is lowered so as to again form the nip portion with the entrance drive roller 83. The stopping nail 85 also lowers down in synchronism with the lowering operation of the entrance driven roller 84 so as to allow the original P to be conveyed to a nip portion between a pair of registration rollers 87.
The original P is conveyed by the pair of registration rollers 87 through a reading position between a contact glass 88 and an exposure roller 89. While being conveyed through the reading position between the contact glass 88 and the exposure roller 89, the original P is scanned with exposure light and thereby an image of the original P is read by a reading device (not shown). The original P is then conveyed outside of the original conveying device to, for example, a sheet discharging tray (not shown), by a sheet-discharging roller 90.
An example of an original conveying device having a configuration as described above is described, for example, in Japanese Laid-open Patent Application Tokukaihei No. 6-230638.
FIG. 8 illustrates another type of an original conveying device, in which an entrance driven roller 84 is not lifted-up/lowered-down as in the aforementioned original conveying device. The original P impinges against the nip portion between the entrance driven roller 84 and the entrance drive roller 83 by being guided by a guide MYLAR 91. The guide MYLAR 91 is arranged such that one end thereof contacts an upper part of the surface of the entrance drive roller 83. The leading edge of the original P is guided by the MYLAR 91 to a position just before the nip portion between the entrance drive roller 83 and the entrance driven roller 84. The configuration of the original conveying device is simplified by eliminating a mechanism to lift-up/lower-down the entrance driven roller 84 and the stopping nail 85.
Typically, the original conveying devices as described above include a plurality of pairs of entrance drive rollers and driven rollers as illustrated in FIG. 9A.
When the original P is manually inserted into the original conveying devices, the original P must be inserted with the leading edge straight and in parallel with a nip line, which is formed by nip portions between the plurality of pairs of entrance drive rollers 83 and entrance driven rollers 84, so that the original P is prevented from being skewed and jammed in the nip portions.
However, when the user manually inserts the leading edge of the original P into the nip portions between the entrance drive rollers 83 and the entrance driven rollers 84, as illustrated in FIG. 9A, the leading edge of the original P may not be in parallel with the nip line when the leading edge impinges against the nip portions. This is caused by the original P being caught by a surface of the entrance drive roller 83, as illustrated in FIG. 9B.
The entrance drive roller 83 is typically made of material having a relatively high frictional coefficient, such as rubber, and the original P is caught by the surface of the entrance roller 83 due to a frictional resistance between the original P and the surface of the entrance drive roller 83. Particularly, when the original P has a relatively thin thickness and has a curl, it is difficult to insert the original P so as to be accurately impinged against the nip line of the nip portions. Typically, the leading edge of such a thin original P impinges and does not form a straight line, as illustrated in FIG. 9A.
A part of the original P that is impinged against the nip portion indicated by point A may be conveyed through the nip portion A antecedently to the other parts of the original P impinging against the nip indicated by points B, as illustrated in FIG. 9A. As a result, wrinkling, jamming, and skewing of the original P tend to occur.
The original conveying device, which is provided with the stopping nail 85 as illustrated in FIG. 7 and in which the entrance driven rollers 84 are lifted up to allow the leading edge of the original P to pass through the nip portions of the plurality of the entrance drive and driven rollers so as to be impinged against the stopping nail 85, can cause the leading edge of the original P to be straight by causing the original P to impinge against the stopping nail 85 and thereby the leading edge of the original P can be accurately align at a right angle relative to the conveying direction of the original P. Therefore, wrinkling, jamming, and skewing of the original P is minimized. However, the original conveying device with the above-described configuration with the stopping nail 85, requires a mechanism to retreat the stopping nail 85 to allow the original P to be conveyed and a mechanism to lift up and lower down the entrance driven roller 84, which increases the manufacturing cost.
Also, if the original conveying device is configured to include the guide Mylar 91 contacting an upper part of the surface of an entrance drive roller 83 to cause the leading edge of the original P to accurately impinge against the nip portion, as illustrated in FIG. 8, the cost of the original conveying device is increased by the provision of the MYLAR 91. In addition, if the original conveying device provided with the guide MYLAR 91 is configured such that an original is first conveyed in a forward direction and then conveyed in the reverse direction for reading an image on the original P, the original P may be jammed at the MYLAR 91 by being interrupted with the guide MYLAR 91 when the original P is conveyed in the reverse direction. In particular, jamming of the original P may occur when the original P is repeatedly conveyed and reversed for making a plurality of copies or when the original P is discharged in the reverse direction.